1. Field of the Invention
This invention relates to zoom lenses, and more particularly to high-magnification range zoom lenses which have at least four lens groups at least three of which are movable for zooming, and which maintain good stability in aberration correction throughout the zooming range.
2. Description of the Prior Art
In a zoom lens system, to change the image magnification while maintaining a constant image plane position, it is necessary to use at least two lens components axially movable in differential relation to each other. Recently, a demand has arisen for zoom lens as of this type with a greatly increased zoom ratio, but in which the bulk and size of the entire system is reduced to a minimum.
If the zoom ratio is increased by increasing the total movement of a variator, the space over which the variator moves must be widened. Thus, the total length of the lens is increased. Such means are, therefore, undesirable.
In general, to obtain a zoom lens of greatly increased zoom ratio while still permitting minimization of the bulk and size, the prior art methods involved strengthening the refractive power of each movable zoom lens group, a reduction in the total movement of the variator, and shortening the axial separations between the lens groups. However, increasing the refractive power of each movable zoom lens group introduces the following drawbacks:
(1) Good correction of aberrations becomes difficult. PA1 (2) The operating mechanism for the lens groups must have a very high accuracy. If this requirement is not fulfilled sufficiently, the optical performance is lowered remarkably.
Increasing the number of movable zooming lens groups to provide a zoom lens with reduced bulk and size is disclosed in Japanese Laid-Open Pat. No. Sho 54-30855 (hereinafter referred to as a "conventional zoom lens").
This conventional zoom lens comprises, from front to rear, a first lens group of positive power, a second lens group of negative power, these two groups constituting a first varifocal assembly, a third lens group of positive power and another lens group arranged either in front, or in rear, of the third lens group, the last two constituting a second varifocal assembly. When zooming from the wide angle to the telephoto positions, the first, second and third lens groups are axially moved in relation to each other so that the first and second varifocal assemblies both exhibit range extending effects. Another effect of this conventional zoom lens occurs when zooming from the wide angle to the telephoto position. Then the second varifocal assembly contributes to an image magnification range which includes unity. On the other hand, by moving the first lens group forward, a magnification changing burden is imposed upon the second lens group, provided that the image magnification of the second lens group does not exceed unity.
Also in the conventional zoom lens, because the zoom ratio is as small as about 3, when the zoom ratio is increased further, the required total movement of the third lens group increases. This produces the following drawbacks with respect to the aforementioned other lens group, which is stationary during zooming, when arranged in front of the third lens group:
1(a) A large space through which the third lens group can move must be created. This makes it difficult to shorten the longitudinal length of the zoom lens.
1(b) The light bundles emerging from the third lens group tend to diverge. Hence the height of incidence of the pencil of rays on the third lens group becomes higher and the diameter of the lens is increased. Also the third lens group produces large aberrations and correcting these aberrations makes the design of the third lens group unavoidably complicated. Increasing the zoom lens ratio also results in the following disadvantages when arranged in rear to the third lens group and the aforementioned other lens is stationary during zooming.
2(a) Because a large air separation between the second and third lens groups is necessarily preserved in the wide angle positions, it becomes difficult to minimize the longitudinal length of the zoom lens.
2(b) Because the total zooming movement of the third lens group increases, and because the pencil of rays strikes the third lens group at a higher height relative to the optical axis, a result similar to that described in paragraph 1(a) occurs. This is not desirable.
These drawbacks of the conventional zoom lens have been found to be ascribable to the fact that the first varifocal lens assembly which would otherwise primarily have a high magnification changing effect is constructed and arranged to provide a low magnifiction range. Moreover a heavy range extending burden is borne by the second varifocal assembly, with the result that the total movement of the movable or third lens group forming part of the second varifocal assembly is increased.
Besides the above, other zoom lenses are known having at least four lens groups of which three are made movable for zooming, as, for example, disclosed in Japanese Laid-Open Patent No. Sho 50-149360 and Japanese Patent Nos. Sho 41-13667 and 44-14877.